Patent GB-694003-A (GB)
Title: Improvements in and relating to quality monitoring indicators
IPC: G07C3/14
Filed: 1951-05-24
Abstract: 694,003. Electric analogue calculating systems. BRITISH THOMSON-HOUSTON CO., Ltd. May 24, 1951 [June 13, 1950], No. 12184/57. Class 37. [Also in Group XL (c)] In a quality monitoring device for continuously indicating the presence or absence of a predetermined proportion of substandard units, voltages are continuously derived respectively proportional to the total number of units counted, and to a statistical function of the total number of substandard units rejected; and a predetermined proportion of the first voltage is compared with the second voltage in a circuit which operates an indicator when the second comparison voltage exceeds the first (or vice versa). On the inspection of a group of units numbering a power of 10 (e.g. 10) produced by a continuous manufacturing process, switch 21 is momentarily closed manually or electronically to impulse a relay 20, energized from rectified A.C. to traverse the double-ended arm 19 of a stepping switch 18 through one of its 10 incremental positions once per group inspected. After 10 steps (e.g. 100 units), the arm reaches the terminal contact through which a relay 16 is impulsed to step the doubled ended contact arm 15 through one position of a bank of contacts 14 of a second stepping switch, respectively connected to a range of tappings of an A.C. fed auto-transformer 10, giving 20 equal voltage increments each corresponding to a count of a power of 10 greater by one than that corresponding to one step of switch 18, so that a maximum count of e.g. 2,000 units may be registered. Intermediate values are registered by a vernier device comprising a second stepping switch 22 operated together with switch 18, having 10 contacts respectively connected to successive alternate taps of the auto-transformer, the voltage on whose contact arm 23 is coupled in series phase addition with the voltage on the contact arm 15 through a 20 : 1 stepdown transformer 26, so that the resultant voltage is proportional to the, number of groups of e.g. 10 units, inspected, representing the running total of units tested, and energizing a potentiometer 29, whose slider 30 is set to represent a predetermined allowable reject rate for the process. On each rejection of a substandard unit, a switch 35 is momentarily closed to impulse a relay 33 and traverse the double ended contact arm 32 through one position of a stepping switch directly counting the number of rejects, whose e.g. 33 contacts are interconnected by impedance elements 31 to form an A.C. energized potentiometer. The voltage on arm 32 represents the running total of rejects, and is combined in phase opposition with the slider voltage of potentiometer 29 and coupled by transformer 36 to the grid of a thyratron 40, whose anode is A.C. energized through an alarm lamp 41, and which strikes to light the lamp when the slider voltage is greater than the reject total voltage or vice versa, to give an indication when the reject rate exceeds the predetermined allowable rate. The successive impedance elements 31'have values which increase progressively with the defect total according to the statistical law of probability, to compensate for the fact that the tolerable reject rate is initially greater than the pretermined allowable rate at low unit tested totals, but progressively approaches more closely thereto as the tested total increases, until equality is reached when the tested total is infinite. Units tested may be counted directly by interposing further stepping switches between switch 21 and relay 20, or the autotransformer may have e.g. 200 taps for directly counting the closure of switch 21, or it may be replaced by a multitapped potentiometer so that the system is operable from a unidirectional voltage source. The thyratron may be replaced by an electromagnetic relay circuit, and the probability compensating non-linearity may be introduced into the total unit counting network.